The pronephros of Scyllium canicula. 247 
in the anterior region of the body of Vertebrate embryos, while the 
pronephros springs from the somatic mesoderm ventral to the meta- 
meric portion of the mesoderm. 
Rickert (Boverı also used RückKErT’s preparations) believed 
that the pronephros in Pristiurus and Torpedo fuses with, and receives 
cells from, the ectoderm, but this has been disproved by van WIJHE 
and Ragr. It is still more unlikely that RÜCKERT saw the prone- 
phrie tubules fuse with one another distally to produce a Sammel- 
rohr, since neither van Wine nor RABz have been able to verify 
this statement. The two last-mentioned authors do not however 
deny that this may happen, and Ragr's description is in favour of 
such a process. Thus RaBz says — “sie giebt sich darin zu er- 
kennen, daß in drei auf einander folgenden Segmenten die laterale 
Urwirbellamelle unmittelbar vor ihrem Übergang in die parietale 
Seitenplatte gegen das Ektoderm etwas vorgewölbt ist.” — A little 
later he says — “Da die einzelnen Urwirbel unmittelbar auf ein- 
ander folgen und dweh keine irgendwie in Betracht kommenden 
Zwischenräume getrennt sind, so bilden die Herwölbungen einen 
kontinuirlichen, sich über vier Segmente erstreckenden Wulst; ich 
will diesen Wulst als Vornierenwulst bezeichnen”. 
One might reasonably expect from Ragr’s account of the earliest 
stages of the pronephros to find some of his figures, either of trans- 
verse or horizontal sections, showing this metameric condition. None 
of Ragr’s figures show this, neither do those of van WisHE, although 
the latter wrote — “In den Stadien mit 35—45 Somiten verwischt 
sich die Segmentirung des Pronephros dadurch, daß sich segmental 
die Kerne in seinem Innern vermehren, wodurch er ganz solid wird. 
Doch kann man bei genauer Beobachtung zwar kein Lumen mehr, 
aber wohl eine Differenz in seinen Querschnitten, welche durch die 
Mitte der Segmente und solche, welche zwischen denselben geführt 
sind, erkennen, da die Zahl der Kerne in den ersteren immer 
geringer ist”. 
Van Wısue’s figures do not illustrate this point: RaBz has 
however, attempted to show the tubule ostia — the first indications 
of the nephrostomes — which are presumably situated opposite the 
middle of the protovertebrae, as for example tab. 13 fig. 8, in 
which the ostium is indicated by the arrangement of the nuclei. 
Surely Rast relied to some extent upon his imagination in deter- 
mining the position of the ostia!! Again, fig. 16 on the same plate 
shows the fourth pronephric ostium (van Wine speaks of three 
